If you want to go back to QuickTime 6 from QT7, use Apple's dedicated
QuickTime 6 reinstaller. Apple also has some QuickTime 7 downgraders, to go back to 7.0.1 from newer versions, in case you're having incompatibilities. If you're not looking to downgrade, then....well, don't uninstall QuickTime! It's an essential part of the OS.
Note that QuickTime is not the same as QuickTime Player. QuickTime Player is just an application like any other. QuickTime is a system-wide service used by a wide variety of applications. Deleting QuickTime Player does not affect QuickTime itself.